[build] Serialize pose detection projects

[jonathanpeppers]

Changes

• Add a reusable mapping for build-order-only project references.
• Serialize pose-detection after pose-detection-accurate so they do not concurrently consume the shared pose-detection-common AAR.
• Use ReferenceOutputAssembly="false" and PrivateAssets="all" so the ordering edge does not affect compilation or NuGet dependencies.

Validation

• Regenerated projects with dotnet cake build.cake --target=binderate --verbosity=minimal.
• Built the pose-detection graph with two MSBuild nodes.
• Packed Xamarin.Google.MLKit.PoseDetection and confirmed PoseDetection.Accurate is absent from the generated .nuspec dependencies.

[Copilot]

Pull request overview

Adds a build-order-only project-reference mechanism to the Binderator project template so specific ML Kit binding projects can be serialized during builds, avoiding concurrent access to shared generated artifacts (notably pose-detection-common).

Changes:

• Introduced a reusable mapping (buildOrderProjectReferences) for “ordering-only” project references keyed by NuGet package ID.
• Added an ordering edge so Xamarin.Google.MLKit.PoseDetection builds after com.google.mlkit.pose-detection-accurate.
• Marked the ordering ProjectReference as non-compilation/non-dependency impacting via ReferenceOutputAssembly="false" and PrivateAssets="all".

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[jonathanpeppers]

@dalexsoto review

[dalexsoto]

The generated build-only edge serializes Accurate before standard pose detection under parallel/static-graph builds without compile or NuGet dependency leakage, and exact-head CI validates both TFMs.